Histones H1(o)a and H1(o)b are the same as CHO histones H1(III) and H1(IV): new features of H1(o) phosphorylation during the cell cycle.

Two histone H1 fractions [H1(I) and H1(II)] and two histone H1(o) fractions (H1(o)a and H1(o)b) have been isolated from butyrate-treated Chinese hamster (line CHO) cells by guanidine hydrochloride gradient chromatography on Bio-Rex 70 ion-exchange resin. The fractions have been identified by electrophoresis and amino acid analyses. Electrophoretic analysis of cyanogen bromide treated H1(o) in long acid-urea-polyacrylamide gels suggests that H1(o)a and H1(o)b differ, at least, within the 20-30 residue fragment(s) removed by the cyanogen bromide cleavage. Shallow-gradient Bio-Rex 70 chromatography indicates that histones H1(o)a and H1(o)b are the same as the respective CHO histones, H1(III) and H1(IV), originally resolved by Gurley and co-workers [Gurley, L. R., Walters, R. A., & Tobey, R. A. (1975) J. Biol. Chem. 250, 3936]. This identification and the phosphate incorporation data of Gurley et al. (1975) reveal new features about H1(o) phosphorylation: (1) following release from G1 arrest, H1(o)a and H1(o)b become phosphorylated in late G1 prior to DNA synthesis; (2) H1(o)a and H1(o)b are phosphorylated at similar rates throughout the cell cycle. These and other data demonstrate that histone H1(o) is phosphorylated in a cell cycle dependent fashion which mimics that of histone H1.